The present invention relates to a stapler for driving staples into an object, such as a sheaf of papers, said stapler comprising a base, a stapler head pivotably connected to the base via a first pin and pivotable between a starting position and a working position in which it is applied against said object when this is placed in the stapler between the base and the stapler head, a reciprocating drive element disposed in the stapler head and adapted to drive a staple into said object during a driving stroke, an operating means which is pivotable about a second pin parallel to the first pin and which is adapted to reciprocate the drive element, and a drive means adapted to pivot the stapler head and the operating means.
A prior-art stapler of this type is schematically illustrated in FIG. 1 of the accompanying drawings.
The illustrated stapler, which is adapted for driving staples into a sheaf of papers, comprises a base 1 and a stapler head 2 pivotably connected thereto. The stapler head 2 is pivotable about a pin 3, fixedly connected to the base 1, between an upper starting position (see FIG. 1) and a working position in which it is applied against a sheaf of papers 4 placed in the stapler between the base 1 and the stapler head 2 and which thus depends on the thickness of the sheaf of papers 4.
An operating means in the form of a pivotable arm 5 is arranged in the stapler head 2. The arm 5 is pivotable about a pin 6 which is parallel to the pin 3 and, like the latter, connected to the base 1. A reciprocating drive element 7, which is arranged in the stapler head 2 in order, during a driving stroke, to expel a staple from a staple magazine 8 arranged in the stapler head 2 and drive the staple into the sheaf of papers 4, is reciprocated by means of the arm 5. To this end, the arm 5 is pivotable between an upper starting position (see FIG. 1) and a lower position in which it is so placed that the drive element 7 has reached the base 1 or, to be more precise, is at a distance from the base 1 that equals the thickness of two sheets of paper, i.e. the thinnest imaginable sheaf to be stapled. The arm 5 is pivoted by a motor-driven eccentric device.
The arm 5 is connected to the stapler head 2 by a spring 9. When the arm 5 is pivoted downwards from the starting position, it entrains, owing to the spring 9, the stapler head 2 until this is applied against the sheaf of papers 4 which, in the example shown in FIG. 1, is much thicker than the smallest imaginable sheaf comprising two sheets of paper. When the stapler head 2 has been stopped by the sheaf of papers 4, the arm 5 continues to move downwards against the action of the spring 9. During this continued downward movement, the arm 5 imparts a driving stroke to the drive element 7, and a staple (not shown) is expelled from the magazine 8 and driven into the sheaf of papers 4.
When the staple has been fully driven into the sheaf of papers 4, the motor-driven eccentric device strives to continue to pivot the arm 5 to its lower position. This is, however, prevented by the sheaf of papers 4 which stops the drive element 7, and consequently the arm 5. This results in a lifting force acting on the pin 6 of the arm 5. To make the stapler work, this pin is therefore connected to the base 1 by a spring 10. Instead of being pivoted further downwards towards the base 1 at its front end, the arm 5 is thus raised against the action of the spring 10 at its rear end. The spring 10 has to be so strongly biased that it does not yield to the staple-driving force, which may be considerable when the sheaf of papers 4 is thick, and permits the pin 6 to be raised before the staple has been fully driven into the sheaf of papers.
The thicker the sheaf of papers 4, the stronger the spring force exerted by the springs 9 and 10 and the stronger the staple-driving force that the arm 5 has to overcome. Thus, the stapling force required increases rapidly as the thickness of the sheaf of papers increases. Consequently, a comparatively strong, and thus expensive, motor is needed for driving the arm 5. Naturally, also the other components have to be dimensioned to withstand considerable stresses, which renders the stapler expensive as well as bulky.
The object of the present invention is to obviate the above drawbacks and provide a stapler in which the stapling force increases comparatively slowly as the thickness of the sheaf of papers increases, thereby enabling the use of a smaller and less expensive motor as well as other less expensive components.